Are Greenhouse Gas Emissions and Soil Chemical Characteristics Affected by Planting Density, Organic Fertilization, and Saline Water Irrigation in Cactus Pear Cultivation?

Abstract

Understanding nitrogen dynamics in arid agricultural systems is essential for mitigating greenhouse gas (GHG) emissions in climate-constrained environments. This study evaluated the effects of planting density, organic fertilization, and saline water irrigation on soil chemical properties, carbon and nitrogen stocks, and emissions of CO₂, CH₄, and nitrous oxide (N₂O) in cactus pear cultivation systems. A 2 × 2 × 2 factorial arrangement was used to test two planting densities (30,000 and 75,000 plants ha⁻¹), two organic fertilizer rates (0 and 30 Mg ha⁻¹), and two saline irrigation depths (0 and 25% of ET₀). Higher planting density increased soil moisture and carbon content while reducing CO₂ and CH₄ emissions. Organic fertilization increased the soil C ratio and phosphorus availability and significantly enhanced N₂O emissions, whereas unfertilized systems showed negative N₂O fluxes. Saline water irrigation reduced N₂O emissions, resulting in negative fluxes (−12.50 µg N m⁻² h⁻¹), indicating potential suppression of nitrification and denitrification pathways. None of the evaluated factors significantly affected soil nitrogen stocks. Total GHG emissions (CO₂-eq) were lower in denser cultivation systems. These results demonstrate that the interaction among high planting density, organic fertilization, and supplementary saline irrigation modulates nitrogen transformations and N₂O emissions in semi-arid soils, highlighting management strategies to mitigate nitrogen-derived GHG emissions in cactus-based agroecosystems.

1. Introduction

The intensification of agricultural systems in semi-arid regions presents major challenges for maintaining soil fertility and reducing greenhouse gas (GHG) emissions, particularly those associated with the nitrogen cycle [1,2]. Nitrous oxide (N₂O) is one of the most potent nitrogen-derived GHGs, and its emissions are strongly regulated by soil moisture, organic matter inputs, salinity, and plant–soil interactions that control nitrification and denitrification processes [3]. In semi-arid environments, where water availability is highly seasonal and soils generally exhibit low organic matter and nitrogen contents, understanding how management practices influence nitrogen transformations is essential [4].

Cactus pear (Opuntia stricta Haw.), a species native to Central America, has gained prominence in semi-arid regions, reaching dry matter yields of up to 71,240.7 Mg ha⁻¹ year⁻¹ [5]. This species is considered an essential component of ruminant feeding systems [6,7,8]. Cactus pear plays a strategic role in these regions due to its high water-use efficiency, drought tolerance, and capacity to contribute carbon and nitrogen inputs to the soil [9,10]. Moreover, cactus pear cultivation systems are highly resilient, making them particularly suitable for the Brazilian semi-arid region. This characteristic is especially important under climate change scenarios, in which water scarcity is expected to intensify and temperatures are projected to increase [11].

Agriculture is a significant source of GHG emissions to the atmosphere, particularly in Brazil; however, several management practices can help mitigate these emissions. The contribution of agriculture to GHG emissions has become a frequent topic in scientific, sociocultural, and political discussions. Nevertheless, it is important to recognize that human, animal, and plant activities all contribute to climate change [12,13]. Human activities undeniably contribute to increasing atmospheric emissions, with carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) being major byproducts of anthropogenic processes [13,14]. Among the principal agricultural practices influencing soil GHG emissions are the application of nitrogen-based fertilizers, both mineral and organic, soil management practices, and crop type [15].

Despite the importance of cactus pear in livestock systems in semi-arid regions, studies evaluating its effects on soil GHG emissions remain scarce. Some evidence suggests that cactus cultivation contributes to soil carbon inputs due to the high concentration of calcium oxalates in cladodes, resulting in the deposition of approximately 0.22 g of carbon per gram of cladode dry matter [16].

In general, low-carbon emission practices are associated with improvements in soil quality, particularly through the accumulation of organic matter, which enhances system resilience by increasing soil water retention capacity. Additional practices that contribute to reducing GHG emissions from agricultural soils include cereal–legume intercropping systems [17], reduced use of synthetic fertilizers combined with manure-based fertilization [18], and the use of saline water (up to 10.6 dS·m⁻¹) to alleviate water stress while reducing soil CO₂ emissions [19].

In this context, we hypothesized that the interaction among planting density, organic fertilization, and saline water irrigation alters soil nitrification and denitrification pathways, thereby modulating N₂O fluxes and soil nitrogen stocks under cactus pear cultivation. Therefore, the aim of this study was to evaluate the effects of planting density, organic fertilization, and supplementary saline irrigation on soil chemical properties, carbon and nitrogen stocks, and soil emissions of N₂O, CH₄, and CO₂ in cactus pear cultivation systems under semi-arid conditions.

2. Materials and Methods

2.1. Description of the Study Area and Environmental Characterization

The experiment was conducted at the Caatinga Experimental Field of the Brazilian Agricultural Research Corporation (EMBRAPA Semiárido), located in Petrolina, Pernambuco, Brazil (09°04′16.4″ S, 40°19′05.37″ W; 379 m altitude) (Figure 1). According to Köppen’s climate classification [20], the regional climate is classified as hot semi-arid (BShw′), with average maximum and minimum air temperatures of 33.46 and 20.87 °C, respectively.

The soil in the experimental area is classified as an Argisol according to the World Reference Base for Soil Resources [21], with medium texture and flat relief. At the beginning of the experimental period, soil samples were collected for physicochemical characterization at depths of 0–0.10, 0.10–0.20, 0.20–0.30, and 0.30–0.40 m (

Table 1. Physical-chemical characterization of experimental area before the experiment implementation.

Chemical analysis
Profile (m)	EC	pH	N	C	P	K+	Na+	Ca2+	Mg2+
	dS/cm		g/kg		mg/dm3	cmolc/dm3
0–0.10	2.63	6.86	0.41	40.08	19.38	0.25	0.57	1.32	0.66
0.10–0.20	2.91	6.81	0.38	39.03	12.90	0.19	0.6	1.15	0.55
0.20–0.30	2.45	6.54	0.37	40.06	7.59	0.17	0.99	1.08	0.55
0.30–0.40	2.70	6.73	0.37	36.17	4.73	0.17	0.78	1.14	0.57
Profile (m)	Al	H+Al	SB	CEC	V	Cu2+	Fe2+	Mn2+	Zn2+
	cmolc/dm3				%	mg/dm3
0–0.10	0	0.47	2.82	3.31	85.94	1.30	49.88	50.8	5.16
0.10–0.20	0	0.53	2.51	3.04	83.05	1.35	46.01	40.37	4.37
0.20–0.30	0	0.76	2.79	3.51	79.24	1.90	46.33	30.63	4.22
0.30–0.40	0	0.82	2.65	3.48	78.04	1.55	48.94	36.38	4.29
Physical analysis
Profile (m)	SD		PD		Porosity		Sand	Silt	Clay
	kg/dm3				%		g/kg
0–0.10	1.45		2.56		42.8		575.75	353.29	70.96
0.10–0.20	1.44		2.55		43.06		616.52	300.15	83.33
0.20–0.30	1.41		2.56		44.51		568.04	312.51	119.4
0.30–0.40	1.38		2.52		45.7		476.01	267.34	256.6

EC = Electrical Conductivity; pH = Hydrogen Potential; N = Nitrogen; C = Carbon; P = Phosphorus; K⁺ = Potassium; Na⁺ = Sodium; Ca²⁺ = Calcium; Mg²⁺ = Magnesium; Al = Aluminum; H+Al = Potential Acidity; SB = Sum of Bases; CEC = Cation Exchange Capacity; V = Base Saturation; Cu²⁺ = Copper; Fe²⁺ = Iron; Mn²⁺ = Manganese; Zn²⁺ = Zinc; SD = Soil Density; PD = Particle Density.

).

Electrical conductivity was determined using a conductivity meter from the saturation paste extract [22]. Soil pH was measured in water at a 1:10 ratio using a potentiometer. The concentrations of available phosphorus (P) and exchangeable potassium (K⁺), sodium (Na⁺), calcium (Ca²⁺), and magnesium (Mg²⁺) were determined according to the methodologies described in the EMBRAPA Soil Analysis Methods Manual [23]. Exchangeable acidity (H+Al), sum of bases (SB), cation exchange capacity (CEC), and base saturation (V%) were calculated according to Teixeira et al. [24]. Total porosity, particle density, and sand, silt, and clay fractions were also determined following the methodology described by Teixeira et al. [24].

The experimental period lasted eighteen months, from October 2021 to April 2023. Throughout this period, rainfall, evapotranspiration, air temperature, wind speed, relative humidity, and global solar radiation were monitored using the Embrapa Semiárido Agrometeorological Station (Figure 2).

Soil moisture was determined by weighing 5.00 g of soil sample, which was subsequently dried in a forced-air oven at 105 °C for 24 h. After drying, the samples were reweighed to obtain dry mass. Soil moisture content was then calculated based on the difference between wet and dry soil weights using the following equation [25]:

$$Moisture in \mathrm{g}/\mathrm{kg}= (weight of dry soil/weight of wet soil) \times 1000$$

2.2. Planting Spacing

The cactus pear clone used in the experiment was IPA-200016/Orelha de Elefante Mexicana (Opuntia stricta Haw.). The cladodes used for planting were obtained from an established cultivation area located at the Caatinga Experimental Field. After harvest, the cladodes underwent sanitary procedures and were subsequently placed in a shaded area for 15 days to allow proper healing before planting.

The experiment was established in an area of 2560 m², previously prepared using a harrow. Planting was carried out with 2.00 m spacing between furrows. Each experimental unit consisted of four rows of cactus pear cultivation, with the two central rows considered the experimental area and the outer rows serving as borders. Planting was performed in double rows, using both sides of the furrow in a “V”-shaped arrangement.

2.3. Treatments and Experimental Design

The treatments consisted of combinations of two planting densities, two organic fertilizer rates, and two saline irrigation depths, arranged in a split-plot randomized block design with four replications, in a 2 × 2 × 2 factorial arrangement. The main plots consisted of two planting densities (30,000 and 75,000 plants ha⁻¹), while the subplots consisted of two organic fertilizer rates (0 and 30 Mg ha⁻¹) and two saline irrigation depths (0 and 25% of reference evapotranspiration—ET₀).

For the planting density of 30,000 plants ha⁻¹, three cladodes were distributed per meter on each side of the furrow, totaling six plants per linear meter, with 33 cm spacing between cladodes. For the density of 75,000 plants ha⁻¹, seven cladodes were distributed on the left side of the furrow (14 cm spacing between cladodes) and eight cladodes on the right side (12 cm spacing between cladodes), totaling 15 plants per linear meter.

Irrigation water was collected weekly during the irrigation period for physicochemical characterization (

Table 2. Physical and chemical characteristics of irrigation water from the artesian well during the experimental period.

Parameters	Average	Maximum	Minimum
Ca2+ (mmol·L−1)	5.18 ± 0.64	6.20	4.26
Mg2+ (mmol·L−1)	17.93 ± 4.52	25.56	14.05
Na+ (mmol·L−1)	12.88 ± 2.68	16.60	9.90
K+ (mmol·L−1)	0.49 ± 0.06	0.55	0.44
SC (mmol·L−1)	36.28 ± 7.16	47.47	30.05
CO32− (mmol·L−1)	0.22 ± 0.07	0.28	0.12
HCO3− (mmol·L−1)	4.81 ± 1.61	7.10	3.29
SO42− (mmol·L−1)	2.32 ± 0.77	3.21	1.63
Cl− (mmol·L−1)	51.60 ± 2.73	53.94	46.56
SA (mmol·L−1)	58.38 ± 2.84	60.86	53.47
pH	6.79 ± 0.22	7.13	6.60
EC (ds·m−1)	4.11 ± 0.47	4.70	3.64
Hardness (mg·L−1)	55.04 ± 7.63	64.60	47.68
SAR	4.09 ± 0.11	4.26	3.98
CaCO3 **	Moderate (3/6)	Hard (2/6)	Soft (1/6)
Classification	C4S2

Mean values are from six collections during the experimental period; maximum value refers to the highest concentration recorded during the assessment; and minimum value to the lowest concentration recorded during the evaluation period. Ca²⁺ = Calcium; Mg²⁺ = Magnesium; Na⁺ = Sodium; K⁺ = Potassium; SC = Sum of cations; CO₃²⁻ = Carbonates; HCO₃⁻ = Bicarbonates; SO₄²⁻ = Sulfates; Cl⁻ = Chlorides; SA = Sum of anions; pH = Hydrogen potential; EC = Electrical conductivity; SAR = Sodium adsorption ratio; CaCO₃ = Calcium carbonate; ** Values within parentheses represent the frequency within the total number of assessments.

). The following parameters were determined according to the methodology described by EMBRAPA [26]: pH, electrical conductivity of the water (EC), cation concentrations of calcium (Ca²⁺), magnesium (Mg²⁺), and sodium (Na⁺), and anion concentrations of chloride (Cl⁻), bicarbonate (HCO₃⁻), carbonate (CO₃²⁻), and sulfate (SO₄²⁻). In addition, the sum of cations (SC), sum of anions (SA), sodium adsorption ratio (SAR), and calcium carbonate content (CaCO₃) were calculated.

According to CONAMA Resolution No. 357, the irrigation water was classified as brackish due to its salinity level, which ranged between 0.5 and 30% [27]. Based on the classification proposed by Richards [28], the water was classified as C4S2, indicating a high risk of salinization and a medium risk of sodification (Table 2). The water was obtained from an artesian well located at the Caatinga Experimental Field, with a depth of 70 m and a flow rate of 1500 L h⁻¹.

The irrigation system consisted of surface drip irrigation using drip tubing equipped with emitters with a flow rate of 1.5 L h⁻¹, nominal diameter of 16 mm, and spacing of 0.20 m between emitters. The system presented a uniformity coefficient of 93% and an average operating flow rate of 0.9 L h⁻¹. The drip lines were installed in the center of the double-row planting arrangement.

Irrigation was performed once per week for 60 min over a five-month period, totaling 20 irrigation events. The applied irrigation depth corresponded to 80% of crop evapotranspiration (ETc), considering an average crop coefficient (Kc) of 0.52 [29]. The irrigation interval was fixed at seven days, and irrigation was applied only during the dry season, always in the late afternoon, as a supplementary water supply strategy.

The organic fertilizer applied to the fertilized plots consisted of goat and sheep manure at a rate of 30 Mg ha⁻¹ year⁻¹ (

Table 3. Chemical composition of manure applied as organic fertilizer (30 Mg ha⁻¹).

EC	pH	N	C	K+	Na+	Ca2+	Mg2+	Al	P	Cu2+	Fe2+	Mn2+	Zn2+
dS·cm−1		----g·kg−1----		-----------------------cmolc·dm−3-----------------------					------------------------mg·dm−3------------------------
0.47	8.1	11.25	38.32	116.87	6	0.02	2.00	1.00	0.00	0.81	13.26	64.95	14.54

EC = Electrical conductivity; pH = Hydrogen potential; N = Nitrogen; C = Carbon; P = Phosphorus; K⁺ = Potassium; Na⁺ = Sodium; Ca²⁺ = Calcium; Mg²⁺ = Magnesium; Al = Aluminum; Cu²⁺ = Copper; Fe²⁺ = Iron; Mn²⁺ = Manganese; Zn²⁺ = Zinc.

).

2.4. Experimental Area Management

The cultivation period lasted thirty months, including twelve months for crop establishment and eighteen months for data collection. Weed control was performed manually throughout the experimental period. Pest control, particularly for carmine cochineal (Dactylopius coccus), was carried out using 1% mineral oil.

2.5. C and N Stocks in Soil

Soil samples were collected along the cultivation rows using an auger to obtain undisturbed samples at depths of 0–0.10, 0.10–0.20, 0.20–0.30, and 0.30–0.40 m, with four replications per treatment, for soil bulk density determination. Soil sampling was performed at the end of the greenhouse gas evaluation period. The undisturbed samples were subsequently used to quantify soil carbon and nitrogen contents.

Soil carbon (C) was determined using the oxidation method with 0.4 mol L⁻¹ potassium dichromate (K₂Cr₂O₇) [30]. Soil nitrogen content was determined using the Kjeldahl method [31]. Soil bulk density was determined using the volumetric ring method described by Teixeira et al. [24]. Carbon (C) and nitrogen (N) stocks were calculated using the following equation:

$$\mathrm{S}\mathrm{t}\mathrm{o}\mathrm{c}\mathrm{k}(\mathrm{M}\mathrm{g}·{\mathrm{h}\mathrm{a}}^{-1})=\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t} \mathrm{C} \mathrm{o}\mathrm{r} \mathrm{N} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}(\%)\times \mathrm{d}\mathrm{e}\mathrm{p}\mathrm{t}\mathrm{h}(\mathrm{c}\mathrm{m})\times \mathrm{d}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}(\mathrm{g}·{\mathrm{c}\mathrm{m}}^{-3})$$

Because the soil in the experimental area was subjected to different management practices, carbon (C) and nitrogen (N) stock calculations were corrected using the equivalent soil mass approach to allow a more accurate comparison among treatments. The correction was performed according to the methodology proposed by Ellert and Bettany [32], using a native Caatinga area located near the experimental site as the reference. The correction was applied to the deepest soil layer (0.30–0.40 m) using the following equation:

Corrected depth (cm) = (average density of the reference area (g·cm⁻³)/average density of the area being corrected (g·cm⁻³)) × depth being corrected

soil sampling in the reference area followed the same methodology used for the experimental area.

2.6. Greenhouse Gas Emissions—GHG

Greenhouse gas (GHG) emissions were monitored at three sampling times during the cactus pear production cycle, at 18 months after establishment, with a seven-day interval between sampling events. Soil gas emissions were collected using static chambers installed at the center of each experimental plot.

The chambers consisted of two sections. The first section was a rectangular galvanized steel base with an area of 2400 cm² (60 cm long × 40 cm wide). The second section consisted of a trapezoidal galvanized steel lid, which was attached to the base during gas collection [33]. Each chamber was equipped with a digital thermometer with a metal probe to monitor internal temperature during sampling, as well as a three-way valve for gas collection using syringes.

During sampling, the chamber lid was attached to the metal base, and gas collection began immediately after chamber closure (time zero), followed by collections at 10, 20, and 40 min after closure. Gas samples were collected using 25 mL polypropylene syringes coupled to a three-way stopcock (Descarpack model) and transferred to previously evacuated glass vials sealed with rubber septa under vacuum (−80 kPa).

Gas sampling was performed at 9:00 a.m. according to the methodology described by Alves et al. [34]. During all sampling events, soil temperature, air temperature, and chamber internal temperature were recorded using digital thermometers. After collection, gas samples were sent to the Embrapa Semiárido Chromatography Laboratory for analysis.

Gas concentrations were determined using an gas chromatograph (Agilent, 7890A, Wilmington, DE, USA) equipped with an injection oven at 60 °C, a flame ionization detector (FID) at 120 °C for CO₂ and CH₄ determination, and a micro electron capture detector (µECD) at 300 °C for N₂O determination.

Fluxes of CO₂, CH₄, and N₂O were calculated based on the increase in gas concentration inside the chamber over time, chamber area, and temperature and atmospheric pressure values recorded during sampling, according to the methodologies described by Signor et al. [35] and Galdino et al. [33]. Methane (CH₄) and nitrous oxide (N₂O) emissions were converted into CO₂-equivalent units and added to accumulated CO₂ emissions to estimate total GHG emissions [35].

Greenhouse gas fluxes were estimated from repeated temporal measurements performed within each experimental unit throughout the evaluation period. Although multiple gas collections were conducted over time, the experimental unit remained the plot, since repeated samplings within the same plot were not considered independent observations.

2.7. Statistical Analysis

Data on soil C and N stocks and soil GHG fluxes were evaluated using one-way analysis of variance (ANOVA) to compare variations between different cactus production systems, according to the following statistical model:

$$Yijklm = \mu + Di + OFj + Ik + Bl + \left(DOF\right)ij + \left(DI\right)ik + \left(OFI\right)jk + \left(DOFI\right)ik + eijklm$$

where: Yijklm = dependent variable to be analyzed; µ = overall mean; Di = effect of planting density; OFj = effect of organic fertilization; Ik = effect of saline water depth; Bl = effect of block; (DOF)ij = interaction of planting density and organic fertilization; (DI)ik = interaction of planting density and saline water depth; (OFI)jk = interaction of organic matter and saline water depth; (DOFI)ik = interaction of planting density, organic fertilization, and saline water depth; eijklm = random error associated with each repetition. Planting density (D), organic fertilization (OF), saline irrigation depth (I), and their interactions were treated as fixed effects, while block was considered a random effect to account for environmental variability within the experimental area.

The data were analyzed using the PROC GLM procedure of the Statistical Analysis System (SAS University Edition) software [36] and subjected to analysis of variance (ANOVA) at a significance level of α = 0.05. When significant effects of planting density, organic fertilization, or saline irrigation depth were detected, means were compared using Tukey’s test at the 5% probability level for Type I error. The standard error of the mean (SEM) was calculated from the original data.

3. Results

No significant interactions were observed between factors, including the triple interaction among planting density, organic fertilization, and irrigation depth (p > 0.05), indicating that these factors acted independently on soil chemical properties (

Table 4. Probability values (p-values) for the chemical assessment of soil in the 0–0.10 m profile under cactus pear cultivation at different planting densities, organic fertilization, and saline water irrigation.

Variables	p-Value
	D	OF	I	D*OF	D*I	I*OF	D*OF*I
Moisture	<0.001	<0.001	0.516	0.766	0.100	0.091	0.592
N	0.501	0.322	0.263	0.259	0.616	0.971	0.636
C	<0.001	<0.001	0.912	0.931	0.080	0.091	0.874
C:N	0.773	0.032	0.950	0.665	0.499	0.691	0.537
pH	0.974	0.038	0.051	0.232	0.616	0.246	0.440
EC	0.009	0.245	0.587	0.166	0.637	0.881	0.997
H+Al	0.106	0.003	0.030	0.100	0.351	0.091	0.997
K+	0.627	0.192	0.889	0.314	0.450	0.613	0.212
P	0.272	0.009	0.031	0.300	0.997	0.196	0.997
Ca2+	0.635	0.871	0.216	0.897	0.336	0.897	0.764
Mg2+	0.500	0.916	0.146	0.896	0.225	0.639	0.997
Na+	0.332	0.014	0.484	0.287	0.084	0.707	0.488
Cu2+	0.169	0.003	0.591	0.092	0.877	0.173	0.997
Fe2+	0.943	0.043	0.572	0.426	0.509	0.142	0.686
Zn2+	0.732	0.284	0.056	0.554	0.205	0.434	0.087
Mn2+	0.895	0.326	0.577	0.480	0.100	0.551	0.194

Soil moisture (g/kg); N = Soil nitrogen (g/kg); C = Soil carbon g/kg); C:N = Carbon to nitrogen ratio; EC = Electrical conductivity (dS/cm); pH = Hydrogen potential; P = Phosphorus (mg/dm³); K⁺ = Potassium (cmolc/dm³); Na⁺ = Sodium (cmolc/dm³); Ca²⁺ = Calcium (cmolc/dm³); Mg²⁺ = Magnesium (cmolc/dm³); H+Al = Potential acidity (cmolc/dm³); Cu²⁺ = Copper (mg/dm³); Fe²⁺ = Iron (mg/dm³); Mn²⁺ = Manganese (mg/dm³); Zn²⁺ = Zinc (mg/dm³); p-Value = Statistical probability; D = Density; OF = Organic fertilization; I = Irrigation depth; D*OF = Interaction between crop density and organic fertilization; D*I = Interaction between crop density and irrigation depth; I*OF = Interaction between irrigation depth and organic fertilization; D*OF*I = Interaction between crop density, organic fertilization, and irrigation depth.

and

Table 5. Mean values for the chemical assessment of soil in the 0–0.10 m profile under cactus pear cultivation at different planting densities, organic fertilization, and saline water irrigation.

Variables	Density (Thousand Plants/ha)		SEM	Organic Fertilization (Mg/ha)		SEM	Irrigation Depth (% ET0)		SEM
	30	75		0	30		0	25
Moisture	53.20 b	66.62 a	2.26	58.88	60.94	2.22	58.88	60.94	2.20
N	2.62	3.05	0.45	2.52	3.16	0.44	2.48	3.20	0.40
C	45.24 b	54.20 a	1.54	35.57 b	63.88 a	1.50	49.60	49.84	1.51
C:N	37.10	35.10	5.43	27.77 b	44.65 a	5.40	36.45	35.97	5.42
pH	6.20	6.20	0.15	6.41 a	5.92 b	0.13	6.42	5.97	0.15
EC	3.31 a	1.84 b	1.20	3.45	1.40	1.21	2.10	3.04	1.20
H+Al	1.10	0.80	0.12	0.70 b	1.30 a	0.13	0.75 b	1.16 a	0.12
K+	0.14	0.16	0.02	0.13	0.19	0.03	0.15	0.16	0.02
P	8.97	6.59	1.49	5.14 b	11.29 a	1.50	5.33 b	10.22 a	1.49
Ca2+	2.34	2.22	0.17	2.30	2.25	0.18	2.44	2.12	0.17
Mg2+	1.22	1.11	0.11	1.17	1.15	0.11	1.28	1.05	0.11
Na+	0.47	0.30	0.12	0.59 a	0.11 b	0.12	0.45	0.32	0.13
Cu2+	1.17	1.45	0.14	0.93 b	1.82 a	0.12	1.37	1.26	0.14
Fe2+	17.46	17.35	1.13	18.91 a	14.40 b	1.14	16.95	17.86	1.13
Zn2+	2.60	2.46	0.27	2.34	2.78	0.28	2.13	2.92	0.27
Mn2+	14.80	15.08	1.47	15.84	13.73	1.48	14.35	15.53	1.47

Soil moisture (g kg⁻¹); N = Soil nitrogen (g kg⁻¹); C = Soil carbon (g kg⁻¹); C:N = Carbon to nitrogen ratio; EC = Electrical conductivity (dS cm⁻¹); pH = Hydrogen potential; P = Phosphorus (mg·dm⁻³); K⁺ = Potassium (cmolc dm⁻³); Na⁺ = Sodium (cmolc dm⁻³); Ca²⁺ = Calcium (cmolc dm⁻³); Mg²⁺ = Magnesium (cmolc dm⁻³); H+Al = Potential acidity (cmolc dm⁻³); Cu²⁺ = Copper (mg·dm⁻³); Fe²⁺ = Iron (mg·dm⁻³); Mn²⁺ = Manganese (mg·dm⁻³); Zn²⁺ = Zinc (mg·dm⁻³); SEM = Standard error of the mean; ^a,b Means followed by different letters on the same line differ statistically from each other by Tukey’s test at 5% probability for Type I error.

).

Planting density significantly affected soil moisture (p < 0.001), carbon content (p < 0.001), and electrical conductivity (p = 0.009) (Table 4). The highest planting density (75,000 plants ha⁻¹) increased soil moisture and carbon content compared with 30,000 plants ha⁻¹ (Table 5). In contrast, EC was higher at the lowest density, indicating greater salt concentration in less dense cultivation systems (Table 5). No significant effect of planting density was observed for the remaining variables, including N, C:N ratio, pH, potential acidity, macronutrients, and micronutrients (p > 0.05) (Table 4).

Organic fertilization significantly affected soil moisture (p < 0.001), carbon content (p < 0.001), C:N ratio (p = 0.032), pH (p = 0.038), potential acidity (p = 0.003), available phosphorus (p = 0.009), sodium (p = 0.014), copper (p = 0.003), and iron (p = 0.043) contents (Table 4). The application of 30 Mg ha⁻¹ of manure increased soil moisture, carbon content, C:N ratio, potential acidity, available p, and Cu²⁺ (Table 5). Conversely, it reduced soil pH, Na⁺, and Fe²⁺ compared with unfertilized systems (Table 5). No significant effects of organic fertilization were observed for N, electrical conductivity, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, or Mn²⁺ (p > 0.05) (Table 4).

The irrigation depth with saline water significantly affected only potential acidity (p = 0.030) and available phosphorus contents (p = 0.031) (Table 4). The application of a saline irrigation depth corresponding to 25% of ET₀ increased potential acidity from 0.75 to 1.16 cmolc dm⁻³ and increased available p from 5.33 to 10.22 mg·dm⁻³ compared with non-irrigated systems (0% ET₀) (Table 5). However, the saline irrigation depth did not significantly affect soil moisture, N, C, C:N ratio, pH, electrical conductivity, K⁺, Ca²⁺, Mg²⁺, Na⁺, Cu²⁺, Fe²⁺, Zn²⁺, or Mn²⁺ contents (p > 0.05) (Table 4).

No significant effects of planting density, organic fertilization, irrigation depth, or their interactions were observed for soil N stock (p > 0.05) (

Table 6. Probability values (p-values) for the carbon (C, Mg/ha) and nitrogen (N, Mg/ha) stock in soil under cactus pear cultivation at different planting densities, organic fertilization, and saline water irrigation.

Variables	p-Value
	D	OF	I	D*OF	D*I	I*OF	D*OF*I
N	0.108	0.273	0.928	0.545	0.083	0.396	0.237
C	<0.001	<0.001	0.600	0.011	<0.001	<0.001	0.009

p-Value = Statistical probability; D = Density; OF = Organic fertilization; I = Irrigation depth; D*OF = Interaction between crop density and organic fertilization; D*I = Interaction between crop density and irrigation depth; I*OF = Interaction between irrigation depth and organic fertilization; D*OF*I = Interaction between crop density, organic fertilization, and irrigation depth.

and

Table 7. Mean values for the carbon (C, Mg ha⁻¹) and nitrogen (N, Mg ha⁻¹) stock in soil under cactus pear cultivation at different planting densities, organic fertilization, and saline water irrigation.

Variables	Density (Thousand Plants ha−1)		SEM	Organic Fertilization (Mg ha−1)		SEM	Irrigation Depth (% da ET0)		SEM
	30	75		0	30		0	25
N	4.48	5.02	0.33	4.48	5.02	0.30	4.73	4.77	0.38
C	238.33 b	295.10 a	1.68	191.83 b	341.60 a	1.97	266.08	267.35	1.82

SEM = Standard error of the mean; ^a,b Means followed by different letters on the same line differ statistically from each other by Tukey’s test at 5% probability for Type I error.

). In contrast, soil C stock was significantly affected by the interactions between planting density, organic fertilization and irrigation (p = 0.009) (Table 6 and Table 7).

The interaction among planting density, organic fertilization, and irrigation demonstrated that systems combining 75,000 plants ha⁻¹, organic fertilization, and saline water irrigation showed the greatest C accumulation throughout the soil profile (Figure 3), indicating a synergistic effect of management practices on soil carbon storage.

Planting density significantly reduced CO₂ emissions (p = 0.016), total emissions (p = 0.004), and CO₂-equivalent emissions (p = 0.004) (

Table 8. Probability values (p-values) for the soil greenhouse gas fluxes and carbon equivalent in cactus pear cultivation at different planting densities, organic fertilization, and saline water irrigation.

Variables	p-Value
	D	OF	I	D*OF	D*I	I*OF	D*OF*I
CO2	0.016	0.277	0.509	0.078	0.361	0.067	0.168
CH4	<0.001	0.356	0.002	0.866	0.014	0.003	<0.001
N2O	0.020	0.002	0.003	0.014	0.049	0.040	0.008
Total emissions	0.004	0.047	0.085	0.910	0.787	0.602	0.571
Eq CO2	0.004	0.046	0.085	0.909	0.787	0.602	0.571

p-Value = Statistical probability; D = Density; OF = Organic fertilization; I = Irrigation depth; D*OF = interaction between planting density and organic fertilization; D*I = Interaction between planting density and irrigation depth; I*OF = Interaction between irrigation depth and organic fertilization; D*OF*I = Interaction between planting density, organic fertilization, and irrigation depth.

), with lower values observed under the density of 75,000 plants ha⁻¹ (

Table 9. Mean values for the soil greenhouse gas fluxes and carbon equivalent in cactus pear cultivation at different planting densities, organic fertilization, and saline water irrigation.

Variables	Density (Thousand Plants ha−1)		SEM	Organic Fertilization (Mg ha−1)		SEM	Irrigation Depth (% ET0)		SEM
	30	75		0	30		0	25
CO2	47.72 a	33.32 b	4.14	43.29	36.81	4.18	38.57	42.46	4.14
CH4	7.36 a	−15.38 b	2.31	−5.69	−8.76	2.33	−12.04 b	−1.98 a	2.31
N2O	−14.28 b	7.24 a	9.68	−13.30 b	9.53 a	9.78	5.46 a	−12.50 b	9.81
Total emissions	56.90 a	−42.22 b	17.51	−51.99 b	47.13 a	16.54	−49.22	−49.90	17.02
Eq CO2	5.69 a	−4.22 b	1.75	−5.20 b	4.71 a	1.65	−4.92	4.99	1.70

CO₂ = Carbon dioxide (mg C m⁻² h⁻¹); CH₄ = Methane (µg C m⁻² h⁻¹); N₂O = Nitrous oxide (µg N m⁻² h⁻¹); total emissions = g m⁻² CO₂-C-eq; Eq CO₂ = Mg ha⁻²; SEM = Standard error of the mean; Conversion used: CO₂ equivalent = 25(CH₄16/12); CO₂ equivalent = 310(N₂O × 44/28). ^a,b Means followed by different letters on the same line differ statistically from each other by Tukey’s test at 5% probability for Type I error.

).

Organic fertilization significantly increased total emissions (p = 0.047) and CO₂-equivalent emissions (p = 0.046) (Table 8). Total emissions increased from −51.99 to 47.13 g m⁻² CO₂-C-eq, while CO₂-equivalent emissions increased from −5.20 to 4.71 Mg ha⁻¹ CO₂-C-eq with organic fertilization (Table 9).

Methane emissions were significantly influenced by the interaction among planting density, organic fertilization, and saline irrigation (p < 0.001) (Table 8 and Table 9; Figure 4).

Higher CH₄ emissions were observed in the systems D30 OF0 I25 and D30 OF30 I25, indicating that saline irrigation under low planting density increased CH₄ fluxes (Figure 4).

In contrast, the lowest CH₄ emissions were recorded in the systems D75 OF0 I0 (−17.58 µg C m⁻² h⁻¹) and D75 OF30 I25 (−24.06 µg C m⁻² h⁻¹), demonstrating that denser cultivation systems promoted greater CH₄ uptake or lower emissions regardless of fertilization management (Figure 4).

Nitrous oxide emissions were significantly affected by cactus production systems (p < 0.05) (Figure 5). The highest N₂O emission was observed in the D75 OF0 I0 system, reaching 82.21 µg N m⁻² h⁻¹, which was 95.88 µg N m⁻² h⁻¹ higher than the D30 OF30 I0 system (Figure 5). The interaction effects observed for N₂O emissions indicate that the combined effects of planting density, organic fertilization, and saline irrigation strongly modulated N₂O fluxes across cactus production systems.

4. Discussion

4.1. Effects of Planting Density, Organic Fertilization, and Saline Irrigation on Soil Chemical Properties

Higher planting density increased soil moisture due to greater soil surface coverage provided by the cactus canopy, which reduced direct soil exposure and consequently decreased evaporative water losses. Similar responses have been reported in denser cropping systems, where increased shading contributes to lower soil temperature and evapotranspiration rates [37,38]. In contrast, organic fertilization did not significantly affect soil moisture, although organic residues are commonly associated with improvements in water retention and reduced deep percolation due to their effects on soil structure [39].

Soil nitrogen concentrations ranged from 2.52 to 3.20 g kg⁻¹, values higher than those reported by Gebretsadik et al. [40] for cactus pear cultivation. Despite this, neither planting density nor organic fertilization significantly altered soil N concentrations. This result may be associated with the high mobility of nitrogen in soil systems and with increased plant uptake under denser cultivation conditions. Donato et al. [41] demonstrated that manure fertilization in cactus pear cultivation increased N accumulation in plant tissues, indicating that part of the applied N may have been rapidly absorbed by plants rather than remaining in the soil. In addition, nitrogen losses through volatilization and other transformation pathways may also have contributed to the absence of detectable differences among treatments [42,43].

The increase in soil carbon under higher planting density was likely associated with greater root biomass turnover and enhanced rhizosphere activity. Cactus pear presents intense root renewal dynamics, particularly under fluctuating soil moisture conditions [44,45], which favors the continuous incorporation of organic residues into the soil. Increased soil carbon may also have contributed to greater water retention, since soil organic matter improves aggregation and water-holding capacity [46]. In the present study, the increase in soil moisture accompanied the increase in soil carbon, reinforcing the close relationship between organic matter accumulation and soil water dynamics.

Organic matter can also be redistributed throughout the soil profile through leaching processes or microbial decomposition. Part of the dissolved organic matter may migrate to deeper layers [47], whereas another portion may be mineralized and released as CO₂ through microbial respiration [48]. In agricultural soils, approximately 58% of soil organic matter consists of carbon, which plays a central role in soil structure stabilization and productivity maintenance [49].

Organic fertilization strongly increased soil carbon content and C:N ratio, demonstrating the important role of manure as a source of organic substrates for semi-arid soils. In addition to contributing directly to carbon accumulation, organic fertilization improves aggregation, nutrient retention, and soil structural stability [50,51,52]. Organic amendments are also capable of mitigating adverse effects caused by irrigation with saline or brackish water [53], while simultaneously improving crop productivity [54] and nutritional composition [55]. These effects are particularly relevant in semi-arid environments, where soils are commonly characterized by low organic matter levels and reduced biological activity [56].

The reduction in soil electrical conductivity under higher planting density suggests greater nutrient uptake efficiency in denser systems. Increased root distribution likely enhanced the absorption of mobile ions, reducing salt accumulation in the soil solution [57]. Furthermore, soil electrical conductivity is closely related to soil moisture and ion concentration [58,59,60], indicating that denser systems may have promoted more balanced water and solute dynamics.

Organic fertilization reduced soil pH while increasing potential acidity. Although manure application is frequently associated with increased pH due to its alkaline components [61], the opposite response observed here may be related to intensified nitrification processes. During nitrification, proton release acidifies the soil solution [62,63], particularly in systems receiving organic N inputs. In addition, microbial decomposition processes and root activity may have intensified proton release and ligand exchange reactions in the soil [64,65,66]. Leaching of basic cations such as Ca²⁺ and Mg²⁺ may also have contributed to increased exchangeable acidity [67,68,69].

The increase in available phosphorus and copper following manure application reflects the nutrient composition of the organic fertilizer and its capacity to enhance nutrient cycling and availability [70,71]. Organic amendments are known to increase total and microbial C, N, and P pools in soil [72], thereby modifying ecosystem stoichiometry and biogeochemical functioning [73]. In the present study, the application of manure altered soil C, N, and P dynamics when compared with the initial soil chemical profile, reinforcing the importance of organic fertilization in nutrient cycling within semi-arid cactus production systems.

Supplementary irrigation with saline water had limited effects on most soil chemical attributes. This response may be associated with the irrigation management strategy adopted in the present study, in which saline water was applied only during periods of greater water deficit. Subsequent rainfall events likely promoted salt leaching and reduced salt accumulation in the soil profile [74,75,76]. Consequently, no substantial increases in soil electrical conductivity or sodium accumulation were observed.

4.2. Soil Carbon and Nitrogen Stocks Under Cactus Pear Cultivation Systems

Nitrogen stocks were not significantly affected by the evaluated management practices. Soil N storage is regulated by multiple interacting factors, including texture, salinity, organic matter content, pH, moisture, and microbial activity [77,78]. Although organic fertilization increased carbon inputs, this effect was insufficient to significantly alter total soil N stocks during the experimental period.

The return of organic residues to the soil represents an important mechanism for increasing soil organic matter and improving ecosystem functioning [79]. Increasing soil carbon stocks is particularly relevant because it contributes to atmospheric CO₂ mitigation and reduces carbon losses from agricultural systems [80,81]. In this context, denser cultivation systems not only increase cactus biomass production [82], but also favor greater carbon inputs into the soil.

Soil carbon accumulation is directly associated with the balance between carbon inputs and outputs [83], which depends strongly on rhizosphere interactions among roots, microorganisms, and soil minerals [84]. Increased planting density likely enhanced belowground carbon inputs through greater root density and turnover [45,85]. Root-derived carbon is considered one of the main contributors to long-term soil carbon stabilization because root exudates and decomposing root tissues stimulate microbial interactions and aggregate formation in the rhizosphere [85,86,87,88].

Greater root density also promotes stronger interactions with soil fauna and microorganisms, accelerating root decomposition and humification processes [86]. Consequently, the transfer of atmospheric CO₂ into soil organic matter is intensified [88,89]. In the present study, higher planting density increased soil carbon stocks, suggesting greater incorporation and stabilization of carbon throughout the soil profile.

Organic fertilization further increased soil carbon stocks, demonstrating its potential as a sustainable strategy for cactus pear production systems. In addition to improving soil fertility and productivity, manure application contributes to climate change mitigation through enhanced carbon sequestration [90,91,92]. Repeated manure applications may also favor nutrient movement and carbon accumulation in deeper soil layers [93], explaining the greater carbon stocks observed along the soil profile.

Saline irrigation can alter soil carbon dynamics by affecting microbial activity and organic matter decomposition [94]. In many cases, saline conditions reduce microbial respiration and carbon stabilization. However, in the present study, supplementary saline irrigation contributed to greater carbon accumulation. This contrasting response may be associated with the intermittent irrigation strategy adopted, which minimized severe salinity stress while improving water availability during periods of drought.

4.3. Carbon Dioxide and Methane Emissions in Cactus Pear Production Systems

The reduction in CO₂ emissions under higher planting density is likely associated with greater soil surface coverage provided by the cactus canopy. Increased canopy coverage reduces direct exposure of the soil to solar radiation, minimizing soil temperature fluctuations and reducing evapotranspiration rates [95,96,97]. Consequently, soils cultivated under higher planting density tend to maintain more stable moisture conditions, which directly influence microbial respiration and carbon mineralization processes.

Organic fertilization did not significantly affect CO₂ emissions, despite increasing soil carbon content. This result suggests that the additional carbon supplied through manure application may have been partially stabilized in the soil rather than rapidly mineralized. Moreover, manure application can stimulate plant growth and photosynthetic activity, increasing carbon fixation and partially compensating for microbial CO₂ production [98,99].

Similarly, supplementary saline irrigation did not significantly alter CO₂ emissions. This response may be associated with the irrigation management strategy adopted in the present study, in which saline water was applied intermittently and only during periods of greater water deficit. Under these conditions, saline irrigation likely improved soil water availability without causing severe salinity stress capable of substantially altering microbial respiration.

The lower CO₂ emissions observed in denser cultivation systems highlight the importance of soil coverage and carbon input dynamics in regulating carbon fluxes in semi-arid agroecosystems. These findings indicate that increasing planting density may contribute to greater soil carbon sequestration and reduced atmospheric carbon losses, reinforcing the potential of cactus pear cultivation systems as climate-resilient production strategies for dryland environments.

Methane emissions were strongly influenced by the interaction among planting density, organic fertilization, and saline irrigation (Figure 4). Systems cultivated under the lower planting density (30,000 plants ha⁻¹) and irrigated with saline water showed higher CH₄ emissions, indicating that saline irrigation under more exposed soil conditions may temporarily reduce methane oxidation capacity.

This response is likely associated with changes in soil microbial activity caused by salinity. Saline conditions can inhibit methanotrophic microorganisms responsible for CH₄ oxidation, thereby reducing methane consumption by the soil [100,101,102]. In addition, salinity may alter carbon and nitrogen mineralization processes and modify electron acceptor availability in the soil environment [103,104].

The ionic composition of irrigation water may also have contributed to the observed responses. The presence of SO₄²⁻ in saline irrigation water can alter microbial metabolic pathways and electron competition processes in the soil, directly affecting methane production and oxidation dynamics [105]. These effects are particularly important in semi-arid soils, where fluctuations in soil moisture and salinity strongly regulate microbial activity.

In contrast, denser cultivation systems (75,000 plants ha⁻¹) presented lower CH₄ emissions or even negative fluxes, indicating greater methane uptake capacity by the soil. Higher planting density likely promoted greater soil coverage, lower temperature fluctuations, increased soil moisture stability, and greater root-derived carbon inputs, creating more favorable conditions for microbial regulation and methane oxidation.

Among the evaluated systems, D75 OF30 I25 presented the lowest CH₄ emission (−24.06 µg C m⁻² h⁻¹), demonstrating a synergistic effect among planting density, organic fertilization, and saline irrigation. This response may be associated with greater organic matter availability in the soil, which acts as a carbon donor and improves microbial stabilization processes. Furthermore, the residual effect of saline irrigation combined with rainfall events may have promoted SO₄²⁻ percolation throughout the soil profile, modifying microbial electron acceptor dynamics and contributing to lower methane emissions [106].

Overall, the results demonstrate that methane fluxes in semi-arid cactus pear systems are regulated by complex interactions among salinity, soil cover, organic matter availability, and microbial activity, reinforcing the importance of integrated management strategies for greenhouse gas mitigation.

4.4. Nitrous Oxide Emissions and Nitrogen Transformation Pathways

Nitrous oxide emissions were also strongly modulated by management practices. Higher planting density increased N₂O emissions, probably because increased soil moisture and carbon availability stimulated denitrification processes [107,108,109]. Under wetter conditions, oxygen diffusion is reduced, favoring anaerobic microsites where denitrifying microorganisms produce N₂O as an intermediate product.

The higher soil carbon concentrations observed under the density of 75,000 plants ha⁻¹ may also have contributed to increased denitrification activity. Carbon availability is a major energy source for denitrifying microorganisms and directly influences N₂O production [109]. Therefore, the greater carbon accumulation observed in denser systems likely stimulated microbial pathways associated with nitrogen transformations.

Organic fertilization also increased N₂O emissions due to the greater availability of carbon and nitrogen substrates for microbial metabolism [110,111]. Organic amendments stimulate microbial activity and provide energy sources for denitrifiers, particularly under conditions of elevated soil moisture. Furthermore, not all applied nitrogen is absorbed by plants, and part of the surplus N may be lost to the atmosphere as N₂O, NH₃, or N₂ [112,113].

The use of saline water is frequently associated with increased N₂O emissions due to changes in soil hydraulic properties and microbial activity [114]. However, this effect was not observed in the present study. The moderate irrigation depth and intermittent application strategy likely avoided severe alterations in soil structure and aeration [115,116], thereby limiting conditions that favor excessive N₂O production.

Several mechanisms may explain the negative N₂O fluxes observed under saline water irrigation. Salinity can inhibit nitrifying microorganisms, particularly ammonia-oxidizing bacteria and nitrite-oxidizing bacteria, thereby reducing substrate availability for denitrification [117]. In addition, elevated concentrations of ions such as Cl⁻ and SO₄²⁻ may alter electron transport processes and microbial metabolism [105,118]. Another possible explanation involves microorganisms carrying the nosZ gene, which are capable of reducing N₂O to N₂ under low-oxygen and moderately saline conditions [119,120]. Together, these mechanisms may explain the negative N₂O fluxes observed under supplementary saline irrigation.

In addition to the conventional expression of greenhouse gas emissions per unit area, a complementary interpretation based on the amount of organic material applied to the soil was considered to improve the environmental assessment of the management system. Considering the application of 30 Mg ha⁻¹ year⁻¹ of goat and sheep manure and the accumulated CO₂-equivalent emissions observed under organic fertilization (4.71 Mg ha⁻¹ CO₂-eq), the estimated emission factor corresponded to approximately 157 kg CO₂-eq per Mg of manure applied. This complementary indicator suggests that, although organic fertilization increased N₂O emissions and total CO₂-equivalent fluxes, the magnitude of emissions per unit of organic residue applied remained relatively moderate under semi-arid conditions. Furthermore, the response cannot be attributed exclusively to manure mineralization, since greenhouse gas fluxes were also influenced by planting density and saline irrigation, demonstrating that the interaction among management practices modulated soil microbial processes and gas exchange dynamics in the cactus pear production systems.

4.5. Implications for Greenhouse Gas Mitigation in Semi-Arid Agroecosystems

Overall, the results demonstrate that cactus pear management practices strongly influence soil carbon dynamics and greenhouse gas emissions in semi-arid environments. High planting density promoted greater soil carbon accumulation and reduced CO₂ and CH₄ emissions, whereas organic fertilization stimulated N₂O emissions through enhanced microbial activity and substrate availability. Supplementary saline irrigation, when strategically applied during periods of water deficit, reduced N₂O emissions without causing substantial deterioration of soil chemical quality.

These findings highlight the potential of integrated cactus pear management strategies to mitigate greenhouse gas emissions while maintaining soil functionality under semi-arid conditions. The combined use of higher planting density, organic fertilization, and supplementary saline irrigation may represent an important strategy for promoting carbon sequestration and improving environmental sustainability in biosaline agricultural systems. Further studies focusing on microbial functional groups, enzymatic pathways, and long-term greenhouse gas dynamics are necessary to better understand the mechanisms regulating negative N₂O fluxes and carbon stabilization in semi-arid soils.

5. Conclusions

Higher planting density enhanced soil carbon storage and reduced CO₂ and CH₄ emissions. However, denser systems also increased N₂O emissions, indicating that greater soil moisture and carbon availability likely stimulated denitrification pathways. Organic fertilization increased N₂O fluxes due to the greater availability of carbon and nitrogen substrates for microbial transformations. Conversely, saline water irrigation reduced N₂O emissions and resulted in negative fluxes, suggesting suppression of nitrification and denitrification processes under moderate salinity conditions.

Although none of the evaluated management factors significantly altered soil nitrogen stocks, they substantially modified nitrogen dynamics and gaseous nitrogen losses. These findings indicate that the integration of high planting density, organic fertilization, and supplementary saline irrigation can modulate N₂O emissions in semi-arid cactus pear systems, providing potential strategies for nitrogen-centered GHG mitigation. Further studies focusing on microbial functional groups and enzymatic pathways are needed to better elucidate the mechanisms responsible for the negative N₂O fluxes observed under saline irrigation.